1. Field of the Invention
This invention relates to multi-mode waveguide filters having temperature compensated dielectric-loaded resonant cavities and to a method of constructing and compensating such filters so that an operating frequency of the filter is substantially constant over a range of temperatures.
2. Description of the Prior Art
When waveguide filters are used on satellites in satellite communications systems, the filters are subjected to harsh environmental conditions. Any components used on a satellite are subjected to stringent weight and volume limitations. It is always desirable to miniaturize satellite components as much as reasonably possible. Usually, less power is required to operate a smaller component than a large component. This allows the satellite to have a smaller amount of power available, which results in a saving of weight and volume or the same amount of power can be made available but can be used to launch and to operate additional components. When satellite components occupy a smaller volume and have a lesser weight, then the satellite can be made smaller and less thrust or power is required to launch the satellite, resulting in substantial cost savings. Alternatively, the space made available on the satellite by reducing the volume and weight of components allows that space to be used for other purposes if the size of the satellite is kept the same. Filters used on satellites are subjected to a wide range of temperatures and often temperature control systems are required on satellites to maintain the temperature of the filters within a certain acceptable narrow range. The temperature control system has a weight and volume that must be taken into account in the overall satellite design. The temperature control system also consumes power as the satellite is operating. If the temperature control system for filters can be eliminated on satellites, substantial cost savings can be achieved.
Temperature compensation of waveguide filters is a desirable result that has been sought for many years. Typically, the material from which a filter cavity is made has a positive coefficient of thermal expansion. As temperature increases, the material expands and the volume of the cavity increases. The operating frequency of the cavity is a function of the cavity's dimensions. As temperature and the volume of the cavity increases, the operating frequency of the cavity decreases. In practice, resonant cavities of filters are constructed from relatively expensive temperature-stable materials such as INVAR nickel steel alloy (hereinafter referred to as "Invar"). However, the use of such materials has not resulted in a wholly acceptable solution to frequency shift. For example, at 12 GHz, it has been found that an Invar cavity shifts 0.9 MHz over a typical operating temperature range for communications satellites. In some applications, a shift of that magnitude is excessive and causes performance to be compromised. For filters used in output multiplexers of communication satellites, a complex and expensive thermal control system is utilized to control the temperature of the cavities making up the filters so that temperature changes can be kept within an acceptable range. When a thermal control system is provided, in addition to the cost of constructing the system, additional power must be made available on the satellite to operate the system. Also, the volume and mass of the thermal control system add greatly to the overall cost of constructing and launching the satellite.
Invar is a relatively heavy material and the use of Invar is therefore disadvantageous where payload weight is an important factor. In addition, Invar has a low level of thermal conductivity. In high power communication satellites, a substantial amount of heat must be dissipated and a thermal control system is necessary on communication satellites to control the temperature of the Invar cavities making up the filters of output multiplexers.
Thus, substantial cost savings can be achieved, even if Invar was continued to be used, by eliminating the thermal control system. Further, if a less expensive or lighter material or a material having a higher degree of thermal conductivity than Invar can be used, further cost savings can be achieved. Temperature compensated filters are known as indicated by the following discussion of references. However, previous filters are much too complex to design or construct; or, the level of temperature compensation available cannot be adjusted after the cavity is constructed; or, they are extremely expensive; or, the temperature compensation features are not sufficiently predictable or repeatable from cavity to cavity; or, the losses are unacceptably high; or, the filters resonate in a single mode.
The Collins U.S. Pat. No. 4,488,132 issued Dec. 11, 1984 describes a temperature compensated resonant cavity where the cavity has a bi-metal or tri-metal end cap so that the end caps expand into or out of the cavity to compensate for the increase or decrease in length of the cavity walls due to variations in temperature. Canadian Patent No. 1,257,349 issued Jul. 11, 1989 granted to Hughes Aircraft Company describes a temperature compensated microwave resonator having a cavity containing a temperature compensating structure that expands or contracts with temperature to minimize the resonant frequency change which would otherwise be caused by the change in volume of the cavity as temperature changes. The Lund, Jr., et al. U.S. Pat. No. 4,287,495 issued Sep. 1, 1981 describes a temperature compensated waveguide where the waveguide is made of a composite material having a plurality of successive plies where one ply has its fiber content aligned parallel to the longitudinal dimension and a second ply has its fiber content aligned parallel to the transverse dimension while third and fourth plies have their fiber content oriented at selected angles relative to the longitudinal dimension such that, as temperature changes, the transverse dimension of the waveguide changes by a sufficient amount to compensate for the change in the longitudinal dimension. The materials suggested are graphite epoxy laminates where the graphite has a negative coefficient of thermal expansion and the epoxy has a positive coefficient of thermal expansion. The cost of a waveguide cavity made from a composite material can be more than ten times the cost of a cavity made from Invar. In all three of the foregoing patents, the design considerations are highly complex. Also, it is sometimes difficult to repeat the thermal compensation results obtained by one cavity with subsequent cavities. Further, when these cavities are constructed, a certain level of temperature compensation is achieved but it cannot be subsequently varied without opening up the cavity and making structural changes to the cavity.
The Bernhard, et al. German Patent No. 2,740,294, disclosed on Mar. 8, 1979, describes a three cavity single mode filter where each cavity has a pin made of NDK ceramic with a negative temperature coefficient. The depth of insertion of each pin into the cavity resonator can be adjusted. The ceramic material is one type of dielectric material and can have a negative or positive temperature coefficient of the dielectric constant.
The Leger, et al. German Patent No. 3,326,830 was disclosed on Feb. 14, 1985 and describes a waveguide circuit which uses a dielectric body having a temperature dependent dielectric constant inserted into a resonator. The patent states that it is possible to compensate the temperature-dependent frequency-response characteristics of a filter using the device. The resonator is a single mode resonator.
The Kell, et al. U.K. Patent No. 1,268,811 was published on Mar. 29, 1972 and describes a microwave device that incorporates a dielectric material that is adjustably mounted within a hole in a dielectric resonator disc so that a frequency of the disc can be adjusted. The dielectric material can be a ceramic and is stated to have a permittivity in the range of 25 to 75. The preferred temperature coefficient of permittivity of the dielectric material is stated in the patent to be in the range from +50 to -100 ppm/.degree.C. The drawings describe a single mode dielectric resonator bandpass filter having five dielectric discs where the dielectric discs are operated at the resonant frequency of the filter.